Many complex machines used in modern industry have numerous sophisticated components which generate heat but can be easily damaged if the temperature of the component is too high. Automatic cooling is extensively used in such machines to prevent temperature rise.
Flow sensors are used in a variety of systems to measure, regulate or control the flow of fluids. An important application of such sensors is to insure that the flow of cooling fluids to a sensitive component does not drop to the point where the component is damaged. Such a fluid flow sensor would therefore measure the flow rate and compare it to a preset threshold. If the flow rate drops below the threshold, and electrical signal is sent to the controls of the system to take predetermined corrective action before serious damage occurs. In sophisticated industrial machines, space is at a premium. Such sensors must be kept small, inexpensive, rugged and sensitive.
In the prior art there are flow sensors which use balls inside transparent tubes. The air flow in the tube must be upward to oppose gravity. The ball rests on a support when there is no flow or low flow. A sufficient flow rate causes the ball to rise to a level where the position of the ball is detected by a light source and light sensor. This type of sensor is large, fragile, and suitable for use in one position only.
Another type of sensor uses a magnet attached to a spring inside the air flow tube. A reed switch outside the tube detects the movement of the magnet due to a change in flow. Friction between the magnet and the walls of the tube causes the flow rate at which the switch is closed to differ from the flow rate at which it is opened by 50%.
There are also flap switches in the prior art which use a hinged flap inside the flow to trigger a microswitch. The mechanical linkage, again, causes friction which causes great differences between the flow rate at which the switch is opened and the flow rate at which it is closed.